Large deposits of bitumen containing tar sands are found in various locations throughout the world. In addition, large deposits of bitumen containing diatomite wherein the bitumen is deposited on predominantly diatomaceous material also exist. As used herein, the term tar sands includes both bitumen containing tar sands and bitumen containing diatomite. The bitumen contained in tar sands is a mixture of hydrocarbons and the bitumen content of tar sands varies over a wide range.
Because of bitumen's hydrocarbon nature, solvent extraction of tar sands is an attractive method for recovery of the crude petroleum. However, spent tar sand must be separated from admixture or suspension in the solvent and dissolved bitumen prior to solvent removal and eventual bitumen recovery. As used herein, such an admixture or suspension containing spent sand is referred to as a solution. Substantially complete separation of spent tar sands is important because for refinery processing the bitumen preferably contains less than about 1.0 weight % solids to avoid catalyst plugging.
The main technical problem in spent sand separation is due to generation of many fine sand particles sized less than 400 mesh during mining and crushing of raw tar sands in preparation for solvent extraction. As used herein, the terms "fine particles" or "fines" mean particles smaller than 400 mesh and all mesh sizes are American Standard sizes. These fine particles interfere with filtration of the solvent bitumen solution because the fines blind the filters. This problem is very acute with bitumen containing diatomite since the diatomite particles are very friable. Thus, crushed diatomite typically can have up to 30 weight % fines.
U.S. Pat. No. 4,240,897, Clarke, "Oil Sands Hot Water Extraction Process", issued Dec. 23, 1980, discloses treating tar sands with hot water and then separating in a primary separation unit a froth which contains bitumen, water and mineral matter, a middlings stream, which is sent to a secondary separation unit for bitumen recovery, and a wet tailings stream. A tailings stream from a froth clean-up unit, which processes the froth from both the primary and secondary recovery units, a tailings stream from the secondary recovery unit, and the wet tailings stream are sent to a filter unit. The wet tailing stream is filtered to form a tailings cake and the tailings stream from the froth clean-up and secondary recovery unit are filtered through the dried cake. To aid filtration of "fines" mineral matter contained in the tailings streams, it is disclosed a flocculant can be added. Clarke is not directed to extraction of bitumen from tar sands with an organic solvent, and does not address the problem of filtering high fines content solutions resulting from extraction of bitumen-containing diatomite with an organic solvent. Further, the streams filtered through the filter cake do not contain bitumen, and are thus more easily filtered. Clarke also does not disclose the average particle size of tailings or the filter cake depth needed to make an operable filter cake.
It is a general object of this invention to provide an improved filtration method for filtering spent tar sands from a bitumen organic solvent solution. Another object is to filter high fines diatomite solutions. Still another object is to provide an improved process for solvent extraction of bitumen from tar sands. Another object is to provide a process for providing bitumen having a solids content which can be refinery processed. Other objects appear below.